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A new phase in the Yb-Se system: high-pressure synthesis and structure refinement of Yb5Se7
Journal of the Less-Common Metals, 157 (1990) L19 - L22
L19
Letter
A new phase in the YbSe refinement of Yb,Se,
system: high-pressure synthesis and structure
K.-J. RANGE*, H. POXLEITNER,
U. KLEMENT and K. G. LANGE
institute of Inorganic Chemistry, ffnjuersity of ~egensburg, ~ni~ersit~tsstr. 31, I)-8400 Regensburg (F.R.C.) (Received September 6,1989)
1. Introduction During an investigation of the high-pressure polymorphism of rare-earth sesquiselenides, a new phase was found in the system Yb-Se. From a detailed single-crystal X-ray diffraction study its composition was determined to be YbsSe,. Yb$e, was formed by high-pressure decomposition of Yb#es at pressures between 20 and 25 kbar and temperatures between 1250 and 1600 “C [ 1.3according to the equation 3Yb$e,
----+ YbSe, + Yb,Se,
It is basically isostructural with Y & [2], space group C2/m. The structure, however, shows some peculiarities, which will be discussed in detail in Section 4. 2. Experimental de tails YbzSes, used as the starting material, was prepared by direct synthesis from a stoichiometric mixture of the elements in evacuated sealed quartz ampoules (800 “C, 6 days). Guinier patterns proved the product to be singlephase Yb$e, with Sc&-type structure. High-pressure experiments were carried out in a modified Belt-type apparatus [ 31, using sintered boron nitride as the crucible material. A sample recovered after quenching from 20 kbar and 1600 “C (reaction time 30 min) consisted of small, black single crystals embedded in a black, microcrystalline matrix. Gandolfi patterns showed the identity of the single crystals with the main phase found in the microcrystalline matrix. A crystal fragment (approximate dimensions 60 pm X 30 I.crnX 30 pm) was used for data collection on an Emaf-Nonius CAD-4 diffractometer (MO Ka, h = 0.71073 A, graphite monochromator in incident beam). Lattice parameters have been refined from 28 values of 25 reflections in the range *To whom correspondence 0022-5088/90/$3.50
should be addressed. @ Elsevier Sequoia/Printed
in The Netherlands
L20
8.4 < 8 < 12.2”. Intensities were measured for 2 < 8 < 30”; w-28 scan technique, scan width (1.0 + 0.34 tan 0)“. Three standard reflections indicated no loss of intensity throughout data collection. Merging of the 1838 collected intensities (sin 0,,,/X = 0.70 A-l; -18 < h < 18, -5 < h < 5, 0 < I< 16) gave 666 unique reflections with I> 2a(I), R(I),. = 0.042, which were used for all calculations (program system SDP 3.1; Enraf-Nonius, 1988). 3. Strut ture analysis
The structure was solved by routine direct methods in space group C2/m. In the least-squares refinement IFI magnitudes were used to refine
positional parameters, occupation factors and anisotropic temperature factors. Convergence was obtained after a few cycles with SOFs for ytterbium and selenium corresponding to the stoichiometry Yb$le, within two standard deviations. Consequently, the SOFs were fixed again at 100% before performing a numerical correction for absorption (program DIFABS [4] ; correction factors ranging from 0.90 to 1.21) and the final anisotropic refinement. Final R = 0.036, wR = 0.028, w-l = a’(F), (A/a),,, < 0.001 in final refinement cycle, 39 variables, S = 1.12. At this stage unusual values for the anisotropic thermal parameters of Se(4) at (2d) +, 0, i and a residual electron density of 5 eAe3 near that atom have been noted. Similar observations have been made by Kim and Franzen [5] during the structure determination of the related compound Y 5_-xSe,. These authors refined and described the structure in space group Cm rather than in C2/m. In the case of YbsSe,, however, the refinement in Cm was completely unsuccessful (very poor convergence, strong correlations, simultaneous refinement of even the isotropic temperature factors impossible). Therefore we decided to retain space group C2/m and to put Se(4) into the split position (4i) with x = 0.5, y = 0, z = 0.5. Anisotropic refinement was possible without any problems, yielding normal temperature factors for Se(4) and k1.4 eAm3in the final difference Fourier map. R-values and variables for all other atoms remained unaffected after simultaneous
TABLE 1 Crystal data for Yb&eT Crystal system Space group a (A) b (8) c (A)
Pi”)
v (A31
2‘
Unique reflections with Z > 2a(Z)
R, wR, S
monoclinic C2/m
13.074(2) 3,9435(S) 12.046(2) 104.77(2) 600.54 2 666 0.036, 0.028, 1.12
L-21 TABLE 2 Atomic coordinates and equivalent isotropic thermal parametersa Atom
x
Y
Wl)
0
Yb(2) Yb(3) Se(l) Set2) S@(3) Se( 4)b
0.69470(6) 0.88492(6) 0.3425(l) 0.2597( 1) 0.9646(l) 0.4931(4)
?Phe equivalent isotropic temperature @B(2, 2) + c*B(3,3) + UCcosfiB(l, 3)). bSpht position, see text.
z
Rx,
0
0
0 0 0 0 0 0
0.80996(8) 0.57812(8) 0.9487(2) 0.6439(2) 0.2155(2) 0.4859(3)
0.68(2) 1.02(2) 1.02(l) 0.71(3) 0.83(3) 1.02(4) 0.58(5)
factors
are defined
as B,, = 413 {a’B(l,
(A*)
1) +
refinement. Crystal data for Yb$e, are given in Table 1 and atomic coordinates and equivalent isotropic thermal parameters in Table 2 [ 61. 4. Discussion Yb$e, is the first rare-earth selenide with an exact 5:7 stoichiometry. Figure 1 shows a projection of the structure along [OlO]. The ytterbium atoms are coord~ated by six (Yb( l), Y b( 3)) and seven (Yb(2)) selenium atoms respectively (Table 3). At the moment it is not clear whether the compound in fact contains divalent ytterbium. The chemical formula Yb,Se, could be written accordingly: YbSe- 2YbzSe, or Yb2+Yb$%$e,. This would
a
Fig. 1. The crystal structure of Yb&Je,, projected along [OlO]. Heights are given in y/100. For clarity, the Se(4) atoms are shown in their unsplitted position.
L22 TABLE 3 Selected interatomic distances (A)
YWHWf
-Se(l) (Yb( l)-Se) ’ Yb(2)-Se(3) -Se(l) -Se(l) -Se(2) (Yb(2)-Se) Yb(3VM3) -Se(4) -Se(2) -Se(4) -Se(2) (Yb(3t-Se)
2.751(2) 2.804(1 j 2.786 2.828( 1) 2.96161) 3.068(Z) 3.079( 1) 2.972 2.748(2) 2.766(3) 2.803(l) 2.815(3) 2.855(2) 2.798
2x 4x 2x 2x 2x
2x
Se(l)--Yb(1) -Yb(2) -Yb(2) (Se0 t_Yb) Se(2)-Yb(3) -Yb(3) -Yb(2) &e(2)-Yb) fM3)_Yb(3) -Yb(l) -Yb(2) (Se(3 )_Yb) Se(4)-Yb(3) -Yb(3) @e(4)_Yb)
2.804( 1) 2.961(l) 3.068(2) 2.920 2.803(l) 2.855(2) 3.079( 1) 2.924 2.748(2) 2.751(2) 2.828( 1) 2.789 2.766(3) 2.815(3) 2.791
2x 2x
2x 2x
2x 2x 2x
agree with the well-known stability of divalent ytterbium. The isostructural compound Y,S, [2], however, shows that a 5:7 stoichiome~ can also be approached with rare-earth elements for which no divalent state is stable.
Acknowledgment The generous support given by the Fonds der Chemischen Industrie is gratefully acknowledged. 1 2 3 4 5 6
K.-J. Range and 11. Poxleitner, in preparation for J. Less-Common Met. C. Adolphe, Ann. Chim., 10 (1965) 271. K.-J. Range and R. Leeb, 2. Naturforsch. Teil 3, 30 (1975) 889. N. Walker and D. Stuart, Acta Crystallogr. Sect. A, 39 (1983) 158. S.-J. Kim and H. F. Franzen, J. Less-Common Met., 138 (1988) L29. A list of observed and calculated structure factors, anisotropic thermal parameters and bond angles have been prepared and may be obtained from the authors (K.-J.R.).
L19
Letter
A new phase in the YbSe refinement of Yb,Se,
system: high-pressure synthesis and structure
K.-J. RANGE*, H. POXLEITNER,
U. KLEMENT and K. G. LANGE
institute of Inorganic Chemistry, ffnjuersity of ~egensburg, ~ni~ersit~tsstr. 31, I)-8400 Regensburg (F.R.C.) (Received September 6,1989)
1. Introduction During an investigation of the high-pressure polymorphism of rare-earth sesquiselenides, a new phase was found in the system Yb-Se. From a detailed single-crystal X-ray diffraction study its composition was determined to be YbsSe,. Yb$e, was formed by high-pressure decomposition of Yb#es at pressures between 20 and 25 kbar and temperatures between 1250 and 1600 “C [ 1.3according to the equation 3Yb$e,
----+ YbSe, + Yb,Se,
It is basically isostructural with Y & [2], space group C2/m. The structure, however, shows some peculiarities, which will be discussed in detail in Section 4. 2. Experimental de tails YbzSes, used as the starting material, was prepared by direct synthesis from a stoichiometric mixture of the elements in evacuated sealed quartz ampoules (800 “C, 6 days). Guinier patterns proved the product to be singlephase Yb$e, with Sc&-type structure. High-pressure experiments were carried out in a modified Belt-type apparatus [ 31, using sintered boron nitride as the crucible material. A sample recovered after quenching from 20 kbar and 1600 “C (reaction time 30 min) consisted of small, black single crystals embedded in a black, microcrystalline matrix. Gandolfi patterns showed the identity of the single crystals with the main phase found in the microcrystalline matrix. A crystal fragment (approximate dimensions 60 pm X 30 I.crnX 30 pm) was used for data collection on an Emaf-Nonius CAD-4 diffractometer (MO Ka, h = 0.71073 A, graphite monochromator in incident beam). Lattice parameters have been refined from 28 values of 25 reflections in the range *To whom correspondence 0022-5088/90/$3.50
should be addressed. @ Elsevier Sequoia/Printed
in The Netherlands
L20
8.4 < 8 < 12.2”. Intensities were measured for 2 < 8 < 30”; w-28 scan technique, scan width (1.0 + 0.34 tan 0)“. Three standard reflections indicated no loss of intensity throughout data collection. Merging of the 1838 collected intensities (sin 0,,,/X = 0.70 A-l; -18 < h < 18, -5 < h < 5, 0 < I< 16) gave 666 unique reflections with I> 2a(I), R(I),. = 0.042, which were used for all calculations (program system SDP 3.1; Enraf-Nonius, 1988). 3. Strut ture analysis
The structure was solved by routine direct methods in space group C2/m. In the least-squares refinement IFI magnitudes were used to refine
positional parameters, occupation factors and anisotropic temperature factors. Convergence was obtained after a few cycles with SOFs for ytterbium and selenium corresponding to the stoichiometry Yb$le, within two standard deviations. Consequently, the SOFs were fixed again at 100% before performing a numerical correction for absorption (program DIFABS [4] ; correction factors ranging from 0.90 to 1.21) and the final anisotropic refinement. Final R = 0.036, wR = 0.028, w-l = a’(F), (A/a),,, < 0.001 in final refinement cycle, 39 variables, S = 1.12. At this stage unusual values for the anisotropic thermal parameters of Se(4) at (2d) +, 0, i and a residual electron density of 5 eAe3 near that atom have been noted. Similar observations have been made by Kim and Franzen [5] during the structure determination of the related compound Y 5_-xSe,. These authors refined and described the structure in space group Cm rather than in C2/m. In the case of YbsSe,, however, the refinement in Cm was completely unsuccessful (very poor convergence, strong correlations, simultaneous refinement of even the isotropic temperature factors impossible). Therefore we decided to retain space group C2/m and to put Se(4) into the split position (4i) with x = 0.5, y = 0, z = 0.5. Anisotropic refinement was possible without any problems, yielding normal temperature factors for Se(4) and k1.4 eAm3in the final difference Fourier map. R-values and variables for all other atoms remained unaffected after simultaneous
TABLE 1 Crystal data for Yb&eT Crystal system Space group a (A) b (8) c (A)
Pi”)
v (A31
2‘
Unique reflections with Z > 2a(Z)
R, wR, S
monoclinic C2/m
13.074(2) 3,9435(S) 12.046(2) 104.77(2) 600.54 2 666 0.036, 0.028, 1.12
L-21 TABLE 2 Atomic coordinates and equivalent isotropic thermal parametersa Atom
x
Y
Wl)
0
Yb(2) Yb(3) Se(l) Set2) S@(3) Se( 4)b
0.69470(6) 0.88492(6) 0.3425(l) 0.2597( 1) 0.9646(l) 0.4931(4)
?Phe equivalent isotropic temperature @B(2, 2) + c*B(3,3) + UCcosfiB(l, 3)). bSpht position, see text.
z
Rx,
0
0
0 0 0 0 0 0
0.80996(8) 0.57812(8) 0.9487(2) 0.6439(2) 0.2155(2) 0.4859(3)
0.68(2) 1.02(2) 1.02(l) 0.71(3) 0.83(3) 1.02(4) 0.58(5)
factors
are defined
as B,, = 413 {a’B(l,
(A*)
1) +
refinement. Crystal data for Yb$e, are given in Table 1 and atomic coordinates and equivalent isotropic thermal parameters in Table 2 [ 61. 4. Discussion Yb$e, is the first rare-earth selenide with an exact 5:7 stoichiometry. Figure 1 shows a projection of the structure along [OlO]. The ytterbium atoms are coord~ated by six (Yb( l), Y b( 3)) and seven (Yb(2)) selenium atoms respectively (Table 3). At the moment it is not clear whether the compound in fact contains divalent ytterbium. The chemical formula Yb,Se, could be written accordingly: YbSe- 2YbzSe, or Yb2+Yb$%$e,. This would
a
Fig. 1. The crystal structure of Yb&Je,, projected along [OlO]. Heights are given in y/100. For clarity, the Se(4) atoms are shown in their unsplitted position.
L22 TABLE 3 Selected interatomic distances (A)
YWHWf
-Se(l) (Yb( l)-Se) ’ Yb(2)-Se(3) -Se(l) -Se(l) -Se(2) (Yb(2)-Se) Yb(3VM3) -Se(4) -Se(2) -Se(4) -Se(2) (Yb(3t-Se)
2.751(2) 2.804(1 j 2.786 2.828( 1) 2.96161) 3.068(Z) 3.079( 1) 2.972 2.748(2) 2.766(3) 2.803(l) 2.815(3) 2.855(2) 2.798
2x 4x 2x 2x 2x
2x
Se(l)--Yb(1) -Yb(2) -Yb(2) (Se0 t_Yb) Se(2)-Yb(3) -Yb(3) -Yb(2) &e(2)-Yb) fM3)_Yb(3) -Yb(l) -Yb(2) (Se(3 )_Yb) Se(4)-Yb(3) -Yb(3) @e(4)_Yb)
2.804( 1) 2.961(l) 3.068(2) 2.920 2.803(l) 2.855(2) 3.079( 1) 2.924 2.748(2) 2.751(2) 2.828( 1) 2.789 2.766(3) 2.815(3) 2.791
2x 2x
2x 2x
2x 2x 2x
agree with the well-known stability of divalent ytterbium. The isostructural compound Y,S, [2], however, shows that a 5:7 stoichiome~ can also be approached with rare-earth elements for which no divalent state is stable.
Acknowledgment The generous support given by the Fonds der Chemischen Industrie is gratefully acknowledged. 1 2 3 4 5 6
K.-J. Range and 11. Poxleitner, in preparation for J. Less-Common Met. C. Adolphe, Ann. Chim., 10 (1965) 271. K.-J. Range and R. Leeb, 2. Naturforsch. Teil 3, 30 (1975) 889. N. Walker and D. Stuart, Acta Crystallogr. Sect. A, 39 (1983) 158. S.-J. Kim and H. F. Franzen, J. Less-Common Met., 138 (1988) L29. A list of observed and calculated structure factors, anisotropic thermal parameters and bond angles have been prepared and may be obtained from the authors (K.-J.R.).